Generally a catalyst for purifying exhaust gases from an internal combustion engine comprises a catalyst in which noble metal components, such as Pt, Pd, and Rh, are deposited on an activated alumina. This catalyst is capable of simultaneously removing hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NOx) and, therefore, has been designated as “three way catalyst”.
This catalyst serves effectively under conditions nearby the theoretical air fuel ratio (A/F), but, has a problem that NOx removal is not sufficient under lean conditions such as an oxygen-rich exhaust gas from a diesel engine.
An attempt for removing NOx under such lean conditions has been proposed which adds a reducing agent, such as ammonia, into the exhaust gases. The use of this process in an automobile, namely a mobile source of noxious exhaust gas, however, has been of no practical use.
Alternatively, a NOx decomposition catalyst has been proposed in which copper is ion-exchanged with zeolite to form a Cu-deposited zeolite catalyst (JP-A-60-125250). This catalyst, however, has a problem that the NOx conversion thereof is low after a high temperature durability test for example at 600° C.
Then, for solving the durability problem a catalyst having copper deposited on β type zeolite has been proposed (JP-A-5-220403). This catalyst, however, has problems that the NOx purification thereof is low at high temperatures (more than 450° C.) and a temperature window range thereof during purification become narrow.